Sulfides in the Environment
Petroleum and natural gas that occur naturally in the geologic formations beneath the earth's surface are often found to contain water, carbon dioxide, sulfur, hydrogen sulfide, and mercaptans. Hydrogen Sulfide and mercaptans (i.e., organic sulfides) have a strong, malodorous “rotten egg” smell, and are toxic. Products containing sulfides, including oil, natural gas, and water, are referred to as “sour.” The levels of organic sulfides in natural gases depend on the source but range from about 0.1 part per million (ppm) to 150,000 ppm.
Both carbon dioxide and hydrogen sulfide are acid gases. They are corrosive in gaseous form and when dissolved in liquids. Transporting and containing acid gases posses a problem for steel piping and containers. Because of this corrosive hazard and the malodorous smell, producers of natural gas must remove sulfides before transport.
Typically, field processing plants are used to remove sulfides prior to transport. Processing plants used for natural gas may use bubble columns, packed columns, tray columns, and/or other methods to absorb the sulfides into a liquid product, by subjecting the gas to countercurrent flow in the presence of sulfide scavengers. Conventional sulfide scavengers are ethanolamines (used in a regenerating plant), triazines, formaldehyde, imines, glyoxal, and combinations, thereof.
Triazines are the most widely used scavengers in recent years, particularly those formed by the reaction of formaldehyde and monoethanolamine. Triazines are a complex chain of aldehyde-like molecules with a calculated molecular weight of about 219 g/mole (i.e., three aldehyde molecules). Triazines react with sulfides to form complex intermediates, and when used to the exhaustion of their reactive imino groups, they form solids. Triazines are popular because of their relative safety compared to formaldehyde, scavenging capacity, and low cost. However, triazines still contain free formaldehyde, which is a known carcinogen and health concern.
Sulfide contamination is endemic to the production of liquid oil and petroleum products. As sweet crude, which is relatively free of sulfur, becomes less available and more expensive, available oil supplies increasingly have some level of sulfur contamination, leading to the formation of hydrogen sulfide and mercaptans. Waste water (including sewage) can also be rich hydrogen sulfide and mercaptans. Heat and acids drive hydrogen sulfide into the air, creating a noxious and toxic environment, e.g., in oil refineries, sewage treatment plants, wastewater treatment facilities, etc.
Hydrogen sulfide and mercaptans are also produced in the putrefaction of organic matter, causing foul smells from brackish ponds, garbage storage areas, and trash collection facilities, and food establishments.
Sulfide Removal Methods
Because of the odor and health problems associated with sulfides, petroleum refiners, chemical cleaning contractors, garbage companies, food establishments, and other industries use methods to control or eliminate hydrogen sulfide and mercaptan emissions. For example, hydrogen sulfide may be scrubbed out of gas mixtures by contacting the gas with caustic alkaline compounds, which remove at least one of the hydrogen atoms, thereby reducing its volatility as a gas and producing a salt. Countercurrent gas flow against a liquid spray or stream through a tray column, packed column or tower is a particularly common method of sulfide removal. This process can also be used with ethanolamines in a recovery system.
Hydrogen sulfide present in gas or water may also be reacted with oxidizing agents such as potassium permanganate but such reactions encourage corrosion in systems and cannot be used with combustible fuels or their vapors.
Hydrogen Sulfide can be reacted with some aldehydes, triazines, imines, or imino compounds to create less odorous compounds. However, such compounds have low solubility in liquid petroleum products (crude oil, gasoline, kerosene, diesel, gear oil, hydraulic fluids, etc.).
The need exists for sulfide scavengers that are soluble in fuels and/or oils, that are efficient in removing hydrogen sulfide and mercaptans, and that are safe to transport and handle.